Electronic tone-masking system for use in the production of colour prints



Sept. 22, 1959 D. M. NEALE ELECTRONIC TONE-MASKING SYST 2,905,755 EM FOR USE IN THE PRODUCTION OF COLOUR PRINTS 2 Sheets-Sheet 1 Filed Feb. 9, 1959 D. M. NEALE NE-MA Sept. 22, 1959 2,905,755 ELECTRONIC TO SKING SYSTEM FOR USE IN THE PRODUCTION OF COLOUR PRINTS 2 Sheets-Sheet 2 Filed Feb. 9, 1959 W 0 -0 Mi 3 a in n Wd 5 M m w 2 fie w M .w. w. H 4. 6 G H v m 0 .w M w .w 0 $53 AMMWWQ MwQESfi l \mwmkw guwkw wgm x ll n.

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United States Patent ELECTRONIC TONE-MASKING SYSTEM FOR USE IN THE PRODUCTION OF COLOUR PRINTS Denis Manktelow Neale, llford, England, assignor to II- ford Limited, llford, England, a British company Application February 9, 1959, Serial No. 792,022

Claims priority, application Great Britain February 19, 1958 6 Claims. (Cl. 178-54) .magnification printing from a colour record.

It is often desirable to print a colour record, hereinafter called the original, on to colour film to produce after suitable processing copies of the original, which may be positive or negative, which will be referred to hereinafter simply as copies. It is well known that in any such process the colouring of the copy tends to be a degraded representation of that of the original. This degradation of colouring is commonly reduced by adjusting the manufacture and processing of the colour film to provide a high contrast, i.e. a characteristic curve of steep gradient. Because a high contrast is used, a reproduction of a scale of grey tones necessarily shows an exaggerated differenee between successive shades of grey. Thus if mid dle tones (corresponding to flesh tones, for example), are reproduced accurately, dark tones such as deep shadows become darker and light tones such as nearwhites and pale skies become lighter. This effect can be disagreeable and in the production of copies from an original positive transparency, an appreciable departure from the desired gradation may occur either in the shadows or in the highlights or in both.

This objectionable feature can be reduced or eliminated by printing from the original in such a way as to reduce its apparent contrast without in any way modifying its colourmg.

Known means for doing this include preparing from the original a grey negative record of the original on panchromatic film. After developing, fixing and drying, this grey negative record is placed in register with the original and light passing through the combination is used to expose the colour film on which the copies are to be printed. This method is expensive and cumbersome in practice and the copies may be marred by scratches and dirt on the grey negative record. These defects are, of course, reproduced in the copy with enhanced contrast.

The need for a separate grey negative record to provide contrast reduction or masking may be eliminated by using a known technique of electronic masking. This electronic system of contrast reduction involves the application of negative feedback to a flying spot scanner as described by R. Theile and H. McGhee, Journal of the British Institution of Radio Engineers, June 1952, pages 325-329. As applied to the printing of colour copies, the arrangement used is shown diagrammatically and in part by Figure 1 of the accompanying drawings. Here deflection circuits 16 cause the electron beam of a cathode ray tube 1 to trace on the phosphor screen 2 a rectangular television-type raster or any other convenient known type of scanning pattern. An image of this scanning pattern is projected by a lens 10 on to the original 6 which is here shown in contact with the colour film 7 which is the copy material on to which the copy is to be printed. Behind the colour film is a photoelectric cell 13 which receives light transmitted by the original 6 and copy material 7 and provides an electrical current which is a. measure of the transmittance of the original 6. This current is applied to an amplifier '17 the output of which is applied to the control grid or cathode of the cathode ray tube in a sense which reduces the brightness of the display at those points projected on to parts of the original having a high transmittance. As a result the display on the cathode ray tube screen takes the form of a negative image of the original, the contrast of this image being a function of the amplifier gain. The foregoing system is hereinafter referred to as an electronic contrast-masking system of the type defined.

It will be appreciated that if, as will usually be the case, the original is being copied on to an integral tripack type of colour film, the light emitted by the cathode ray tube must contain substantial amounts of energy in the three bands corresponding to the wavelength bands to which the three emulsions of the colour film respond. Usually these three bands are in the red, green and blue parts of the spectrum. The relative energies required in the three bands must be balanced so that an original which represents a record of a neutral grey scale will produce a duplicate in which the steps of the scale are also grey.

To obtain satisfactory spectral emission from the cathode ray tube it may be necessary to use a screen compounded of a mixture of two or three phosphors. Alternatively or additionally it may be necessary to interpose between the cathode ray tube and the colour film a colour modifying filter or filters. For example, it is well known that to minimise the colour degradations due to overlapping of dye absorption bands in the original and overlapping of emulsion response bands in the colour film, printing is best performed by a light source providing energy at only three discrete wavelengths, these wavelengths coinciding approximately with the peaks of the dye absorption curves. An approximation to such a spectrum may be produced by a cathode ray tube in which the screen comprises a mixture of red, green and blue phosphors each emitting over a relatively narrow band of wavelengths. In practice, however, a better result may be obtained by using a highly efiicient phosphor in conjunction with selective filters. Despite the use of such combinations of phosphors and filters, the light used for exposing the colour film must still be balanced as described above, i.e. substantial proportions of its energy are contained in each of three bands of the spectrum designated red, green and blue. The term balanced light as hereinafter used will be understood to be defined in this way.

It is well known that in an integral tripack colour film the subtractive dyes used to form the colour image often approximate only indiiferently to the ideal dyes which are required. In particular, the magenta and cyan dyes, which should transmit blue freely, often have substantial absorptions in the blue region. As a result of these deficiencies, subtractive colour films commonly reproduce saturated blues at an unduly low brightness. When a copy is made from the original a furtherloss in brightness of blues is apt to be experienced. Depending on the dyes used by the colour process in question, any particular colour or pair of colours (red, green or blue) may suffer loss of brightness in this way.

It is anobject of the present invention to provide a method and apparatus for counteracting the loss of brightness in certain colours arising in the copying of subtractive colour films. According to the presentinventiomin the production of colour copies by means of an electronic contrast masking "system of the type defined, the printingof the duplicate -is effected by substantially balanced light as hereinbefore defined and the photocell is selected, or its response modified by the interposition of colour filters, tOfPlOdllCG a substantial responseorily to 'part of that portion of the spectrum of the printing light to whieh the=emulsion or emulsions of the copy-material "respond. In a particular form of the invention the photoeell is insensitive to, or made not to receive, the red emission of-the printing light.

The invention may be further defined as a process'for 'the productionof'colourcopies which comprises causing the electron beam of a'cathode ray 'tube to trace a scanninggpatternon the-phosphor-screen of such tube, projecting an-image of this scanning pattern on to a colour record, usingthis 'light'toprint on to a photographic copy material, collecting light from said'copy material on a photo-electric cell, feeding the response of said cell to the'control system "of the cathoderay tubeto reduce the brightness of-the display on the screen at those parts corresponding tothose'parts ofthe colour record having a relatively low optical density whereby the display on the cathode ray tube takes the form of a negativeimage 'of'the 'colour record, the light emitted by the screen containing substantial proportions 'of its energy in those parts "of the spectrum required'forprinting, and providing *that'the light to which'the photoelectric cell is caused ito respond is in respect of part only of that portion of the spectrum of the light used for printing to which -the photographic'copymaterial is responsive.

'T he invention will bexfurther explained withreference to the remaining figures of the accompanying drawings in which:

."Figure' 2 isa diagrammatic representation of theloptical system seen in'front elevation.

Figure 3 is a diagrammatic representation of the optical "system seen insideelevation.

Figure 4 is a curve'indicating'the quality of light .emitted by the cathode ray tube.

Figure 5 illustrates the characteristic of the type of contrast mask ideally required.

Figure 6 illustrates the type of contrast masking characteristics readily provided in practice.

'tIn Figures'2 and 3 there is shown a cathode ray tubel the internal face of which carries a phosphor screen 2 comprising a mixture of a small proportion of zinc sulphide emitting greenlight under electron excitation and "greater proportions of silver-activated zinc sulphideemitfting desaturatedblue-green light and cadmium phosphate emitting red 'light. On removal of electron excitation the light emission from either zinc sulphide phosphor falls in about 10 microseconds to 1/2 ofits emission under excitation. The corresponding fafterg'low time for the cadmium phosphate component is about milliseconds. These three phosphor components combine to .emit, when continuously excited, light of a quality indicate'd'by the plot against wavelength of energy per unit wavelength band, as shown in Figure 4. The spectral energy distribution of this light is not satisfactory for the printingof copies and accordingly colour selective filters 3,4,.5 are interposed in the optical path in aplane which 'is not 'imaged'in the plane of the original 6, or the colour I'film 7 on to which the-original is to be printed. .Prefer- 'ably the filters 3, 4 and 5 are arranged, as shown, close to the -lens mount 8 which houses the projection lens systemf9. .10. They serve to balance the spectral energy distribution of the light used iforprinting the copy.

Light passing throughboth the original 6 and the colour film 7 enters an "integrating light-guide 1'1, and 'is so conveyed to'the photocathode 12, of an end-window multiplier photoclblS. Between the colour film 7 and the photocathode 12 is a further colour-selective filter or filters I4 and15. If the photocell 13 shows substantial response to'the'redlight emission of thecadmiumphos- ,phate component in the phosphornfthe cathode ray tube 1, one of these filters should be of a red-absorbing nature. In -this-way it is provided that the photocell cannot respond to saturated reds in the original and these are accordingly printed in the same way as blacks, i.e. they receive maximum light from the cathode ray tube and are accordingly relatively brighter.

il'hersecond-of-thesetwofilters maybe a blue-absorbing filter which :renders I the ,rphotocell insensitive to saturated blues in the original. Thus, while both filters are used the contrast =mask -.-is derived entirely from the green transmission of the original. As a'result, "the brightness of saturated ,greens is :restricted by the contrast mask, but that of saturated blues and reds is not.

These "filters are equivalent to ya green transmitting filter, but it is convenientto'keep 'the two components separate, so that rthe absorptions -;can be separately adjusted to give maximum transmission in the green.

The effective mask density, plotted against density of *the "original, "should have :a characteristic of 'the form shownin Figure'S. '(T he derivation of this curve'is described in fThe Reproduction of'ColourjR. W. "G. Hunt, pages -74, Fountain Press, 1957.) Using a linear .feedback amplifier and assuming a cathode ray tube with "linear modulation characteristic, the system of electronic masking described above provides an equivalent masking characteristic as shown inFigure 6. A better approximation to 'the required characteristic is 'obtaiuediin'the foregoing embodiment ofthe invention by using a tlio'de to "limitthe amplitude of the feedback signal so that the brightness of the raster is'the same at all points corresponding to parts of .the original havinggreen-light densities lessthan about 027 density unit.

'It will be understoodithat the foregoing example of 'the invention, 'inwhich the gphotocellis made .not to receive, or to receive a reduced amount of, the re'dantl "blue regions :of'the spectrum, ispurely illustrative and essentially of value for the colourfilm materials which are currently available on the ,market. Changesin .the absorption characteristics of the dyes in the colour film would necesistate variations'in the selection of theresponse 'to be derived from the photocell.

1. A process for theproduction of .colour copies which comprises causing the :electron .beam of a cathode ray tube toitrace a scanning pattern on the phosphorscreen of such tube, projecting an image of this-scanningpattern on to a colour record, using .this light to print,on vto..a photographic copy material, collecting light from .said

.copymaterial on a photoelectriecell, feedingtheresponse energy in thoseparts of the spectrum required: for printing,

and providing .that theilight to which the photoelectric cell is caused to respond .is in respect of ,part only .of that portion of the spectrumof the light .used forprinting to which the photographic .copy material is responsive.

,2. Aprocessfor theproduction of colour copies which comprises causing the electron beam of .a cathode ray tube totrace a scanning pattern on the-,phosphorscreen of such tuhe, projecting animage of this scanningpattern on toa .colour record, using .thislight to print..on.to.a photographic copy material, collecting light from said copy material ona photoelectric cell, feeding the response of saidcell to the control system .of the cathode ray ,tubeto reduce the brightness of the display. on thetscreen .at thoseparts corresponding to those parts oftthetcolour record having .a relatively low optical density whereby the display on .the.cathode.ray tubetakesthe form .of .anegativeimage of the-colour record, the.l ight.emitted by "the screen .containing .substantial proportions flOfi iiS energy in those parts of the spectrum required for printing, and providing that the photoelectric cell is responsive to part only of that portion of the spectrum of the light used for printing to which the photographic copy material is responsive.

3. A process for the production of colour copies which comprises causing the electron beam of a cathode ray tube to trace a scanning pattern on the phosphor screen of such tube, projecting an image of this scanning pattern on to a colour record, using this light to print on to a photographic copy material, collecting light from said copy material on a photoelectric cell, feeding the response of said cell to the control system of the cathode ray tube to reduce the brightness of the display on the scren at those parts corresponding to those parts of the colour record having a relatively low optical density whereby the display on the cathode ray tube takes the form of a negative image of the colour record, the light emitted by the screen containing substantial proportions of its energy in those parts of the spectrum required for printing, and providing that the light accepted by the photoelectric cell is received via a colour filter absorbing part of the portion of the spectrum of the light used for printing to which the photographic copy material is responsive.

4. A process for the production of colour copies which comprises causing the electron beam of a cathode ray tube to trace a scanning pattern on the phosphor screen of such tube, projecting an image of this scanning pattern on to a colour record, using this light to print on to a photographic copy material, collecting light from said copy materital on a photoelectric cell, feeding the response of said cell to the control system of the cathode ray tube to reduce the brightness of the display on the screen at those parts corresponding to those parts of the colour record having a relatively low optical density whereby the display on the cathode ray tube takes the form of a negative image of the colour record, the light emitted by the screen containing substantial proportions of its energy in those parts of the spectrum required for printing, and providing that the photoelectric cell is insensitive to red light, that being light to which the photographic copy material is sensitive.

5. A process for the production of colour copies which comprises causing the electron beam ,of a cathode ray tube to trace a scanning pattern on the phosphor screen of such tube, projecting an image of this scanning pattern on to a colour record, using this light to print on to a photographic copy material, collecting light from said copy material on a photoelectric cell, feeding the response of said cell to the control system of the cathode ray tube to reduce the brightness of the display on the screen at those parts corresponding to those parts of the colour record having a relatively low optical density whereby the display on the cathode ray tube takes the form of a negative image of the colour record, the light emitted by the screen containing substantial proportions of its energy in those parts of the spectrum required for printing, and providing that the light accepted by the photoelectric cell is received via a colour filter absorbing red light, that being light to which the photographic copy material is sensitive.

6. A process for the production of colour copies which comprises causing the electron beam of a cathode ray tube to trace a scanning pattern on the phosphor screen of such tube, projecting an image of this scanning pattern on to a colour record, using this light to print on to a photographic copy material, collecting light from said copy material on a photoelectric cell, feeding the response of said cell to the control system of the cathode ray tube to reduce the brightness of the display on the screen at those parts corresponding to those parts of the colour record having a relatively low optical density whereby the display on the cathode ray tube takes the form of a negative image of the colour record, the light emitted by the screen containing substantial proportions of its energy in those parts of the spectrum required for printing, and providing that the light accepted by the photoelectric cell is received via filters absorbing red and blue lights, these being lights to which the photographic copy material is sensitive.

No references cited. 

